Melanocytes are embryonically derived from the neural crest. These cells migrate to the skin during fetal development, sit on the basal lamina of the epidermis and interdigitate with basal cells via dendrites. Melanin is produced in the Golgi apparatus of the cell and this pigment is packaged (melanosomes) and delivered to keratinocytes and the hair follicle. In some cold-blooded vertebrates (frogs, fish, and reptiles), the cells synthesize melanosomes but do not pass it on to other cells. The melanosomes can move back and forth from the nucleus to the peripheral processes (dendrites) and this mechanism of dispersion and aggregation gives these animals the ability to change skin color from dark to light or vice versa. In cold-blooded vertebrates melanocytes are called melanophores.
α-, β-, and γ- Melanocyte-Stimulating-Hormone (MSH) and adrenocorticotropin (ACTH) are melanocortin receptor (MCR) peptide agonists derived (in humans) from post-translational processing of pro-opiomelanocortin (POMC). MCR are seven-transmembrane domain G-protein coupled receptors first discovered in 1992. Five subtypes have been cloned and named MCR-1 to MCR-5. There is consensus that MCR-1 exists on normal and neoplastic melanocytes and activation of these receptors results in increased melanogenesis (via G-protein stimulation, cAMP accumulation and tyrosinase activation). The MCR-4 receptor is implicated in body weight regulation. For example, inventors Gu et al. describe using melanocortin-4 receptor as a target to treat body weight disorders by modulating the activity of that receptor, WO 97/47316, published Dec. 18, 1997.
Melanoma is a tumor originating from unrestrained proliferation of melanocytes, which are pigment cells residing mainly in the epidermis. This tumor has an annual incidence in the United States of about 35,000 cases, with a mortality of approximately 7300 deaths (for 1997). The incidence of melanoma has been increasing significantly (with a 300 percent increase in the past 40 years). Currently, the lifetime risk of melanoma in the U.S. is approximately one percent. But in some countries such as Australia and New Zealand the lifetime risk is as high as 1/15 or 6.7%.
The reason for the increased incidence of this disease is uncertain but may stem from greater recreational sun exposure, especially early in life, and a just released study suggests that sunscreen does not protect against skin cancer, including melanoma. Individuals most susceptible to development of melanoma are those with fair complexions, red or blond hair, blue eyes, and freckles and who are poor tanners and easy sunburners. Other factors associated with increased risk include family history of melanoma (approximately one in ten melanoma patients have a family member with melanoma) and the presence or excess of atypical moles.
Malignant melanoma is usually first detected as a change in size or shape of a pigmented area of the skin and confirmed by histological examination of the biopsy specimen. The five-year survival for localized disease (clinical stages I and II) is about 85 percent. For clinical stage III (clinically palpable nodes that contain tumor cells), a five-year survival of about 50 percent is noted when only one node is involved and about 15 to 20 percent when four or more nodes are involved. Five-year survival for clinical disseminated disease (stage IV) is less than five percent. Fortunately, the majority of melanomas are diagnosed in clinical stages I and II and melanomas less than 0.76 mm thick are usually cured by surgical removal (five-year survival rates range from 96 to 99 percent). On the other hand, metastatic melanoma to organs such as brain, liver, and lung, is associated with survival of less than one year.
Current treatment of the disseminated disease is usually palliative to improve the quality of life. Surgical excision of a single metastasis to the lung or to accessible brain sites can be associated with prolonged survival. Radiation therapy can provide local relief for recurrent tumors or metastatic sites. Patients who have advanced regional disease isolated to a limb may benefit from localized intra-arterial limb perfusion with chemotherapeutic agents. However, chemotherapy has a response rate of only 20 to 25 percent and rarely induces complete remission. The lack of response to traditional cancer treatments has led to many trials using agents such as retinoids, high-dosage chemotherapy with autologous bone marrow transplantation, antipigmentary agents, and antibodies conjugated to isotopes, drugs, and toxins. More recently, immunotherapy with interleukin-2 and α-interferon has been used. Marginally improved response rates have resulted from these experimental methods.
Wei and Thomas discovered that a number of dynorphin A peptides had the unusual property of preventing the increased vascular permeability of small blood vessels that occurs after tissue injury. This anti-inflammatory property of dynorphin A peptides and certain des-Tyr dynorphin A compounds and analogs is described in U.S. Pat. No. 5,482,930, issued Jan. 9, 1996.
Some MCR antagonists have been found in the agouti proteins (named after the South American rodent), which are 131 (mouse)/132 (human) amino acid proteins elaborated by hair follicle melanocytes and by brain tissue. The elaboration of agouti-signaling protein (ASP) determines the hair color of fur-coated animals. Agouti also antagonizes MCR-2 to MCR-5, but analyses of the binding coefficients do not indicate the clearcut characteristics of competitive antagonism which is found for MCR-1. (Yang et al., Mol. Endocrinol., 11, pp. 274-280, 1997.) Siegrist et al. recently showed that ASP binds to the MCR-1 with an almost identical affinity to that of α-MSH, and that it had antiproliferative action with a half-maximal effective concentration of 13 nM. Agouti protein was also found to induce MC down-regulation. They also showed that ASP will inhibit the α-MSH-stimulated growth of B16-F1 mouse melanoma cells in culture. Siegrist et al., J. Recept. Signal. Transduct. Res., 17, pp. 75-98 (1997); Siegrist et al., Biochem. Biophys. Res. Commun., 218, pp. 171-175 (1996). However, an agouti-related protein (ARP) which was recently cloned from non-epidermal tissues had little effect on MCR-1 at concentrations up to 100 nM, although it did cause a dose dependent inhibition for MCR-3 and MCR-4. Ollmann et al., Science, 278, pp. 135-138 (1997).